1. Field of the Invention
This invention relates generally to electronic test and measurement, and, more particularly, to circuits for measuring current drawn from or provided by power supplies, parametric measurement units, and other instruments used in electronic test and measurement, including automatic test equipment.
2. Description of Related Art
A need commonly arises in electronic test and measurement (T&M) for measuring currents delivered to various loads. T&M includes automatic test equipment, or “ATE.” ATE systems, or “testers,” are complex, usually computer-driven, electronic systems for verifying the operation of electronic devices, circuits, or assemblies.
FIG. 1 is a high level block diagram of a tester 100. A host computer 110 runs a test program developed for testing a unit under test, or UUT 120. The host computer 110 interactively communicates with power supplies 112, PMUs 114, and other instruments 116. These instruments provide stimuli to and/or monitor responses from the UUT 120 via an interconnect 118. Examples of testers are well known in the art, and include the Catalyst™, Tiger™, Panther™, FLEX™, and UltraFLEX™ test systems, designed by Teradyne, Inc. of North Reading, Mass.
One electronic instrument that involves current measurements is the parametric measurement unit, or “PMU.” As is known, a PMU is an electronic instrument for applying regulated voltages and/or currents to UUTs, and for measuring voltages and/or currents from UUTs. Tasks performed by PMUs typically include forcing a voltage to a node or pin of a UUT and measuring the resulting current that flows (FVMI). They also include forcing a current to the UUT and measuring the resulting voltage manifested as a result of that forced current (FIMV).
FIG. 2 shows a conventional arrangement for effecting FVMI testing of a UUT 230 with a PMU. An amplifier, such as a power op amp 210, receives a programming signal, FV, and produces a corresponding output voltage, Vout. The programming signal is generally designated by the test program. Current flows through the UUT in response to Vout. The current may be the leakage current of a semiconductor device, for example. An ammeter 212 is coupled in series with the output of the power op amp 210 and provides a measure of current (MI) flowing into the UUT 230. The measured current may be reported back to the test program to determine whether it falls within acceptable limits.
FIG. 3 shows a conventional arrangement for effecting FIMV testing of a UUT 330 with a PMU. This arrangement may be established by reconfiguring the same PMU shown in FIG. 2. Here, a power op amp 310 receives a current programming signal, FI, from the test program and produces a corresponding output current, Iout. An ammeter 312 is coupled in series with the output of the power op amp 310 to provide a measure of current. The current is fed back to the power op amp 310, allowing negative feedback to maintain the desired current. The output current Iout flows through the UUT and manifests a voltage (MV). The voltage may be reported back to the test program to determine whether it meets the test limits.
FVMI and FIMV arrangements are directly relevant to power supplies as well as PMUs. Power supplies delivering output voltages generally operate in FVMI mode. They not only generate regulated output voltage, but also provide a measure of resulting output current. Some power supplies act as current sources. In these instances, they may operate in FIMV mode, regulating output current while providing a measure of output voltage. Indeed some power supplies can operate in both FVMI and FIMV modes, transitioning between them as the load changes.
The ammeter is a critical design element for both PMUs and power supplies. FIG. 4 shows a more detailed view of a conventional ammeter 212/312. The ammeter 212/312 includes a bank of resistors 412a-n connected between the ammeter's input and output. The ammeter also includes a bank of switches 414a-n. The switches 414a-n allow the resistors 412a-n to be individually selected. For example, closing the switch 414a (the top switch) and opening the other switches 414b-n ensures that all current flowing from the input to the output of the ammeter passes through resistor 412a (the top resistor).
The individually selectable resistors 412a-n equip the ammeter with different current ranges. The resistors 412a-n are commonly provided with decade spacing, e.g., 1-ohm, 10-ohms, 100-ohm, and so forth. By selecting a different resistor, a different current range is configured. Low value resistors (e.g., 1-ohm or 10-ohms) allow high current measurements, whereas high value resistors (e.g. 10 k-ohm or 100 k-ohm) allow accurate measurements of low currents. The resistors are preferably very stable over time and temperature and have precisely known values.
The current flowing through the ammeter 212/312 can be determined by measuring the voltage across the selected resistor and applying Ohm's Law. The ammeter includes a differential amplifier (op amp 420 and resistors 422, 424, 426, and 428) to perform this function.
The ammeter 212/312 includes other components to avoid errors. As current flows through the selected resistor, a voltage drop appears across its associated switch (one of 414a-n). For large currents, this voltage drop can contribute a substantial error. A second bank of switches 416a-n is included to prevent this voltage drop from appearing in current measurements. Only one switch 416a-n is closed at a time. If the resistor 412a is selected, for example, only the switch 416a (leftmost switch) is closed. Any voltage drop across the switch 414a that results from current flow through the switch is therefore not included in the signal sent to the differential amplifier. Another component that avoids errors is the voltage buffer 418. The voltage buffer 418 is disposed between the switches 416a-n and the differential amplifier to ensure that no current is drawn from the output side of the ammeter, where it would contribute a direct measurement error. The voltage buffer 418 also ensures that no significant voltage drop appears across any of the switches 416a-n, which would also contribute an error.
Although the ammeter 212/312 performs well and with high precision, we have recognized that it is not always optimal. Consider the case in which an instrument (e.g., a power supply or PMU) is configured for FVMI operation and programmed to output a desired voltage into a UUT having an unknown resistance. An initial current range may be selected; however, the optimal current range may be different from the initial setting. Since changing current ranges involves opening and closing certain of the switches 414a-n and 416a-n, changing the current range can cause abrupt changes in the output impedance of the instrument. Changing the current range can thus disturb feedback signals of the instrument, causing transient responses that can damage the UUT and take significant time to settle. If the load is inductive, a sudden change in output impedance may cause a voltage spike that can damage the UUT or the instrument itself. A different type of problem can occur when the switches 414a-n and 416a-n are implemented with analog switches. Changing switch positions with the instrument connected to the load can cause charge to be injected from the analog switches into the load, disturbing and potentially damaging the load.
Performance of the ammeter 212/312 may also suffer when the impedance of the UUT suddenly changes while a voltage is being applied. Consider the case in which an ammeter 212/312 is used in a tester power supply to provide current to a UUT, such as a microcontroller. As is known, some microcontrollers have a low current “idle” mode in which they consume very little power; however, they can switch rapidly to a high current operating mode. If the ammeter is configured for measuring the microcontroller's idle current, the optimal range will involve a high value resistor 412, such as 100 k-ohms or more. With such a high value resistor selected for the ammeter, the power supply will have a high output impedance. A subsequent transition of the microcontroller to its operating mode will then cause the power supply's output voltage to drop. Output voltage will fall, essentially to zero, and the microcontroller will cease to operate. Testing of the microcontroller will then be compromised.
To some extent, this problem can be avoided by providing an alternative, conductive path around the ammeter. For example, a pair of oppositely oriented zener diodes 440 and 442 may be connected in series between the input and output of the ammeter 212/312. With this arrangement, the zener diodes may conduct (one forward and one reverse) when the microcontroller is switched from idle mode to operating mode. Output voltage regulation may eventually be reestablished with output current supplied through the zener diodes. However, regulation does not generally resume until after a deep and sometimes lengthy drop in output voltage, which typically will be enough to reset the microcontroller and compromise the test. Another, typically smaller, transient will often ensue when a high current range is later selected and the zener diodes turn off.
What is needed, therefore, is an ammeter that does not suffer from damaging output transients when different current ranges are selected or when the load changes impedance.